An example of such steam-heating apparatus is known from Japanese patent application laid-open No. Hei. 7-328423. In this steam-heating apparatus, as shown in FIG. 6, a reaction vessel 1 is provided as a heat exchanger. The reaction vessel includes, as a heating section thereof, a jacket section 2, to which a steam supply pipe 3 for receiving supply of heating steam and a condensate recovering unit 6 for discharging condensate produced as a result of heating are connected. More particularly, the steam supply pipe 3 is connected to an inlet of a steam ejector 5 and the jacket section 5 as a heating section is connected to a suction chamber 13 formed in an outer periphery of a nozzle communicated with the inlet of the steam ejector 5. Also, a shutoff valve 47 is connected to an outlet communicated with the suction chamber 13.
In operation, by opening the shutoff valve 47, air remaining within the jacket section 2 will be drawn into the suction chamber 13 to evacuate the inside of the jacket section 2. Thereafter, by closing the shutoff valve 47, low-pressure steam is supplied into the jacket section 2, so that an object in the reaction vessel 1 may be heated by low-temperature steam of 100° C. or lower.
With the conventional steam-heating apparatus described above, the apparatus is capable of steam-heating an object with the air remaining at the initial stage of the heating process being removed by suction. However, this apparatus cannot exhaust air which may be gradually accumulated with progress of the heating process, so that the apparatus becomes unable to maintain a predetermined heating temperature precisely. Specifically, when the heating apparatus is placed under the evacuated condition below the atmospheric pressure, atmospheric air may be inadvertently drawn through in the respective joints of the apparatus, thus leading to gradual accumulation of air inside the heating section. Moreover, the heating steam supplied to the apparatus may contain some air therein. Therefore, disadvantageous accumulation of air inside the heating section occurs not only in the initial stage of the heating process but also during the subsequent heating stage thereof.